Metal coating pretreating agent

ABSTRACT

Corrosive resistant, coated laminate consisting of metal substrate, anti-corrosive layer and paint layer, which is characterized by the fact that the anti-corrosive layer is formed by a composition containing, as essential components, at least one hexa-valent chromium containing chromium compound having a solubility of 20 to 10 -5  and aqueous silica and/or aqueous resin as binder, and as optional component, a water soluble chromium compound having a solubility of more than 20, and the paint layer is free from chromium compound. The laminate is excellent, inter alia, in adhesion of coating and anti-corrosive property. The invention also provides a coated metal plate having excellent corrosion resistance and continuous spot weldability.

This application is a division of Ser. No. 07/103,013 filed Sept. 30,1987, now U.S. Pat. No. 4,853,285, which is a division of Ser. No.869,857 filed May 28, 1986, now U.S. Pat. No. 4,719,038, which is acontinuation-in-part of Ser. No. 687,004 filed Dec. 27, 1984, nowabandoned.

FIELD OF INVENTION

The present invention relates to a laminate consisting of metalsubstrate, anti-corrosive layer and paint layer and being excellent in,inter alia, adhesion of coating and anti-corrosive property, andpreparation thereof.

The invention also concerns a method for forming such anti-corrosivelayer, materials to be used therefor and coating composition capable ofresulting in a coating with excellent properties and especiallycontinuous spot weldability.

BACKGROUND OF THE INVENTION

A metallic sheet such as zinc, iron and aluminium plates is usuallyapplied with a coating from the esthetic and other functional points ofview and often subjected to a pretreatment to improve adhesion of thecoating and anti-corrosive property.

As the aforesaid pretreatment, there are such methods as phosphatetreatment, chromate treatment, anodizing method and the like, and amongthem, particular attention is directed to the chromate treatment becausea highly anti-corrosive coating can be obtained therewith.

Various proposals on this chromate pretreatment technique have beenheretofore made for further improving the anti-corrosive property andadhesion of the coating.

These proposals include, for example, a technique for forming insolublecoating by controlling the ratio of hexavalent chromium to the totalchromium in the chromate solution (e.g. Japanese Patent Publication No.18217/64, ibid 37567/79, a technique for using, as binder, colloidalsilica or water dispersible silica, in a coating type chromatecomposition to improve chromium pick-up (e.g. Japanese patentapplication Kokai No. 10834/77, ibid 92339/78), a technique for using awater soluble or dispersible resin for the same purpose (e.g. Japanesepatent application Kokai 30235/77, ibid 49029/78), a technique foradding phosphoric acid, fluorine salt or the like as reaction promotor(e.g. Japanese Patent Publication No. 26525/69, ibid 42135/77), atechnique for adding salts of such metals as Ni, Co, Mn, Zn, Ba and thelike as film modifier (e.g. Japanese patent application Kokai No.105486/81, ibid 34178/83) or a technique wherein the metallic substrateis first treated with a chomate solution containing a reaction promotorand then with a coating type chromate liquid.

However, in these methods, there is indeed an increase in initialcorrosion resistance, but none of the methods can solve the essentialproblems inherently possessed by the chromate coating, of degradation incorrosion resistance caused by thermal change or degradation incorrosion resistance with time due to the reaction between activechromium in chromate coating and other constituents thereof.

On the other hand, compounding of anti-corrosive pigment such as solidchromate pigment, lead pigment and the like having comparatively lowersolubility, to a coating composition for which a higher corrosionresistance is desired has been practiced under certain circumstances.However, there is a problem that chromium, lead and the like are liableto be dissolved out of the coating and hence the corrosion resistance bedecreased with time.

Moreover, though it is not so serious as compared with the case of usingwater soluble chromium compound, such chromate pigment may react withother constituents of certain paints, and thus there are coatings andhence compositions which cannot include such chromate pigment. Even if acoating composition can stand such pigment, there exists a limit in theemployable chromate pigments and in some cases, it will be essential touse a considerably larger quantity of pigment, and these facts willbring about additional, complicated and difficult problems in thecombination of pigment and coating composition.

Under the circumstances, it would be an immeasurable advantage if atechnique is established wherein a metal surface is effectivelyprotected by a coating which is excellent in thermal resistance and iscapable of fixing chromium compound on a metal surface that can neverreact with the coating resin and can exhibit excellent corrosionresistance for a longer duration of time, and on which any type ofcoating composition may be freely applied. The inventors, havingendeavored to find a good solution to this problem, have found an ideal,metal coating pre-treating composition comprising a hexa-valent chromiumcontaining compound having a specified range of solubility, a specifictype of binder and an optional water soluble chromium compound. Theinvention has been made on the basis of this finding.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a metal coatingpre-treating composition capable of resulting in an anti-corrosive layeron zinc, iron or aluminium plate, which may improve adhesion of thecoating, and is free from the degradation of corrosion resistance withtime caused by change of properties under heating and/or reaction withpaint resin.

Another object of the invention is to provide a metal coatingpre-treating composition capable of resulting in an anti-corrosivecoating on zinc, iron or aluminium plate, which may exhibit, even whenapplied with a coating composition containing no chromium or leadcompound, excellent adhesion of coating and corrosion resistance. Otherobjects of the invention will be apparent to those skilled in the artfrom the following description of the invention and claims.

These and other objects of the invention can be attained with thepresent composition comprising at least one hexavalent chromiumcontaining chromium compound having a solubility of 20 to 10⁻⁵ andcompatible binder, the content of metal chromium being 10⁻¹ to 100 g/land the content of said binder being 10⁻² to 500 g/l on solid basis.

In a particularly preferable embodiment, said composition may furtherinclude a water soluble chromium compound in an amount, in terms ofmetallic chromium, of 10⁻¹ to 100 g/l. The total amount of chromiumcompound(s) in terms of metallic chromium is more than 10%, andpreferably more than 20% of the total non-volatile residue.

In this specification, the term "solubility" denotes "amount in grams ofthe compound dissolved in 100 grams of water at 20° C.".

PREFERRED EMBODIMENTS OF THE INVENTION

Hexa-valent chromium containing chromium compounds with solubility of 20to 10⁻⁵ used in the present invention are exemplified by calciumchromate (typical member with a solubility of 20 to 10), zinc chromateand strontium chromate (typical members each having a solubility of 10to 10⁻¹), and barium chromate and lead chromate (typical members eachhaving a solubility of 10⁻¹ to 10⁻⁵).

The said compounds may include a mixture of hexa-valent chromiumcompound and other metal, or reaction product of hexa-valent chromiumcompound and an organic substance, providing it has a solubility of 20to 10⁻⁵.

Since a chromium compound having a solubility of more than 20 isexcessively reactive to the resinous component of the coating, there isonly insufficient corrosion resistance with time, of the coating.Whereas, with a chromium compound whose solubility is less than 10⁻⁵,the corrosion resistance is quite low and hence it is unable to attainthe object of this invention.

Among the abovementioned chromium compounds, calcium chromate having asolubility of 20 to 10 is especially useful in giving a higher corrosionresistance with an active substrate, or in a slightly humid, corrosiveatmosphere. However there is shortcoming of lacking in durability ofenhanced corrosion resistance. Though lead chromate, a typical memberhaving a solubility of 10⁻¹ to 10⁻⁵, is excellent in giving an enhancedcorrosion resistance in a highly humid corrosive atmosphere, itsactivity is rather poor under a slightly humid condition. Whereas, suchmembers as zinc potassium chromate, strontium chromate and the like dopossess appropriate solubility, i.e. 10⁻¹ to 10, and hence can exhibitexcellent corrosion resistance under all corrosive circumstances,including highly corrosive circumstances. These chromium compounds maybe used alone or in combination of 2 and more depending on the type ofsubstrate material, coating composition and film performance required.

In the present pre-treatment composition, the abovesaid chromiumcompound should preferably exist in a range, as metallic chromiumconcentration, of 10⁻¹ to 100 g/l. This is because at a level of lessthan 10⁻¹ g/l, it is unable to attain the objects and effects of thepresent invention, and at a level of more than 100 g/l, stability of thepre-treatment composition will get worse. In the present invention, theabovesaid chromium compound is used together with the compatible binder.Such binders include aqueous silica, aqueous resin and the like.

Examples of aqueous silica are colloidal silica or water dispersiblesilica having a mean diameter of less than 100 mμ. More specifically,mention is made of colloidal silica such as Snowtex O (colloidal silica,manufactured by Nissan Kagaku K.K., solid 20%, mean diameter 20 mμ, pH3.0).

Snowtex N (pH 9), Snowtex OL (mean diameter 40 to 50 mμ, pH 3), and asexamples of water dispersible silica, there are Aerosil 300(manufactured by Nippon Aerosil Co., mean diameter less than 100 mμ),Aerosil TT-600 (manufactured by Nippon Aerosil Co., mean diameter of 0.1to 0.3 mμ), Imsil A 108 (manufactured by Illinois Mineral Co., meandiameter 5 mμ) and the like. The abovementioned aqueous resin mayinclude both water soluble and water dispersible resins havingcompatibility with the chromium compound.

Examples of water soluble resin are polyvinyl alcohol, alkyl hydroxyalkyl cellulose, polyacrylic acid and its derivative, polyvinylamide andits derivative, polyvinyl pyrrolidone, polyvinyl methylether, reactionproduct of maleic anhydride and vinyl or acryl compound and the like,and examples of water dispersible resin are alkyd resin, phenol resin,amino resin, vinyl acetate emulsion polymer, styrene butadiene seriessynthetic latex polymer, acrylic emulsion polymer, natural and syntheticrubber emulsion polymer and the like.

Such binders may be used alone or in combination of 2 and more and areuseful in adhesively fixing the abovesaid chromium compound to the metalsurface.

Among the abovesaid binders, much preference is given to aqueous silicabecause of its capability of resulting in maximum corrosion resistance.

The binder component is advantageously selected in a range of 10⁻² to500 g/l on solid basis. In case of aqueous silica, it is preferablydetermined within said range, since at a lower level than 10⁻² g/l, nosatisfactory effect of the invention can be obtained and at a higherlevel than 500 g/l, no additional effect can be expected therewith. Inthe case of aqueous resin, it is preferably selected in a range of 10⁻¹to 50 g/l. This is because, at a lower level than the abovesaid lowerlimit, no satisfactory effect of the invention can be obtained and at ahigher level than the abovesaid upper limit, there is a trend thatstability of the composition will become worse.

The present pre-treatment composition is preferably adjusted to pH 1 to10, because excessive metal dissolution is liable to occur at the timeof film-forming if the pH is lower than 1, whereas there is anundesirable alkali corrosion of metal, if the pH is higher than 10, andin either case, no satisfactory effect of the invention can be expected.In order to adjust the pH of the pre-treatment composition, variousinorganic or organic acids, and volatile bases such as ammonia water,amine and the like may be used.

The present pre-treatment composition may be applied in a usual way suchas, for example, by dipping method, spray method and roll coatingmethod, on zinc, iron or aluminium metal surface, and subjected, withoutwater washing, to drying.

The drying conditions may be appropriately selected, providing themaximum metal surface temperature is limited to 60° to 200° C. As to theapplication amount of said pre-treatment composition, it is generallyselected in a range of 0.05 to 5 g/m² as dried film weight. This isbecause, if the amount of said composition is less than the lower limit,no satisfactory corrosion resistance can be obtained and when the saidamount is over the upper limit of 5 g/m², no additional effect can beexpected.

As to the metal chromium content, it is preferred to be within a rangeof 0.01 to 1 g/m².

Thus obtained protecting film is stable against thermal degradation,shows excellent corrosion resistance, and can exhibit long-lastinganti-corrosion property because of no occurence of the reaction withresinous component of clear or upper coat.

The inventors have also found that a further improvement in both initialand successive corrosion resistance can be obtained when a water solublechromium compound is added to the present pretreatment composition in anamount, in terms of metal chromium, of 10⁻¹ to 100 g/l.

Therefore, in the most preferred embodiment of the present invention,there is provided a metal coating pre-treatment composition comprising10⁻¹ to 100 g/l (as metal chromium) of at least one hexa-valent chromiumcontaining chromium compound having a solubility of 20 to 10⁻⁵,10⁻¹ to100 g/l (as metal chromium) of at least one water soluble chromiumcompound, the hexa-valent chromium being preferably more than 10% byweight of the total soluble chromium, and 10⁻² to 500 g/l (on solidbasis) of binder material which is compatible with chromium.

The term "water soluble chromium compound" denotes the chromium compoundhaving a solubility of more than 20, and preferably more than 40.

The abovesaid water soluble chromium compound may be any such compoundused in heretofore known chromate treating compositions, including CrO₃,Cr(NO₃)₃. 9H₂ O, Cr₂ (SO₄)₃. 18H₂ O, and other various chromates orbichromates.

They are used alone or in combination of 2 or more.

In this invention, such water soluble chromium compound is used in anamount of 10⁻¹ to 100 g/l (on metal chromium basis), so as to givehexa-valent chromium of more than 10% by weight of the total watersoluble chromium. This is because, at a lower level than 10⁻¹ g/l, nosatisfactory effect of water soluble chromium compound is obtained,whereas at a higher level than 100 g/l, no additional effect can beexpected therewith.

The amount of hexa-valent chromium occupied in the total amount of watersoluble chromium is also important, because at a lower level than 10% byweight, no appreciable improvement in corrosion resistance has beenobtained. Preferably, the hexa-valent chromium should be 50 to 90% byweight of the total water soluble chromium. No additional effect can beexpected by the increase in hexa-valent chromium content over 90% byweight.

When use is made of the present pre-treatment composition containing awater soluble chromium compound, the amount of said water solublechromium compound in the coating should preferably be in a range (asmetal chromium) of 0.01 to 0.5 g/m² for the attainment of sufficientprotecting effect and adhesion of the coating.

The present pre-treatment composition may further include, as reactionpromoter, at least one of phosphoric acid, fluoric acid, fluosilicicacid, fluoboric acid, fluozirconic acid, fluotitanic acid and saltsthereof, thereby increasing the protecting ability of chromate coatingand corrosion resistance thereof.

Regarding the amount of said reaction promotor, it has been found thatthe most preferable range is about 5 to 20 g/l for phosphoric acid(phosphate), and about 0.03 to 10 g/l for fluoric acid, and complexfluoric acid (salts).

Thus treated metal substrate may be applied with any kind of paintcompositions including clear coating. They may be of aqueous, solvent orpowder type. The metal substrate may also advantageously be subjected toelectrostatic coating or electrodeposition coating as desired.

Since there is no need of adding chromium pigment or the like to thetop-coat composition, the resinous component and other ingredients maybe freely selected for the said top-coat. When the pre-treatmentcomposition includes water soluble chromium compound, even if thereoccurs certain reaction between the said chromium compound and coatingresin, the composition likewise contains less reactive and less solublechromium compound, which will give enough anti-corrosive effect andnever flow out of the coating, and thus one may expect a long lastingcorrosion resistance therewith.

The anti-corrosive coating laminate of the present invention consists,therefore, essentially of metal substrate with or without anti-corrosivechemical pretreatment, anti-corrosive layer of the compositioncomprising at least one hexa-valent chromium containing chromiumcompound having a solubility of 20 to 10⁻⁵, binder material composed ofaqueous silica and/or aqueous resin, and optional water soluble chromiumcompound having a solubility of more than 20and paint layer which issubstantially free from chromium compound.

As already stated, any of the coating compositions may be freely usedfor the said paint layer and there is no need of adding chromate pigmentor the like for anti-corrosive purpose, to the composition.

Among various coating compositions, particularly preferable are thecompositions based on epoxy resin, vinyl butyral resin, oil-freepolyester resin and/or carboxylated polyethylene resin, because theirfilm-forming temperatures are relatively low and they are excellent inadhesion of the coating when applied with a top-coat.

The film thickness of the paint layer may be appropriately selected in arange of 1 to 10μ.

In the abovesaid laminate, the anti-corrosive layer is expressed interms of a single layer composed of the present pre-treatmentcomposition.

However, when water soluble chromium compound is used as in the mostpreferable embodiment of the invention, the anti-corrosive layer may becomposed of two separate sub-layers, and the water soluble chromiumcompound is preferably positioned in the inner sub-layer to minimize theflow-out of said compound and expect further improvement in corrosionresistance of the composite laminate.

Thus, in the present invention, there is provided a pre-coating treatingmethod for a metal substrate comprising treating the substrate with areaction type chromate treating agent containing water soluble chromiumcompound and a reaction promotor, subjecting the same to an optionalwashing and/or drying, contacting the substrate with a chromate solutioncontaining hexa-valent chromium containing chromium compound having asolubility of 20 to 10⁻⁵ and aqueous silica and/or aqueous resin, andimmediately drying the same.

The abovesaid reaction type chromate treating composition and thetreating method per se are conventional. As the water soluble chromiumcompound, mention is made of CrO₃, Cr(NO₃)₃. 9H₂ O, Cr₂ (SO₄)₃. 18H₂ O,and other chromates and bichromates. At least one of the said members isused in an amount of 10⁻¹ to 100 g/l (as metal chromium) and hexa-valentchromium is preferably included in an amount corresponding to more than10% by weight of the total soluble chromium.

Examples of reaction promotor are phosphoric acid (ca. 5 to 20 g/l),fluoric acid, fluosilicic acid, fluoboric acid, fluozirconic acid,fluotitanic acid and salts thereof (0.03 to 10 g/l in terms of F),sulfuric acid (ca. 0.1 to 10 g/l), hydrochloric acid (ca. 0.1 to 10 g/l)and the like.

Degreased and cleaned metal substrate is first contacted with the saidreaction type chromate liquid by spraying, dipping or the like.

The amount of the chromate coating applied is usually selected in arange of 10 to 250 mg Cr/m². If the chromium coverage is less than 10mg/m², no sufficient effect of the invention can be attained and if itis over the upper limit of 250 mg/m², adhesion of the coating will belowered.

The bath temperature is usually fixed in a range of room temperature to50° C., and treating time is set in a range of 1 to 30 seconds.

If necessary, thus treated substrate is washed with water and dried.When dry off is required, it is carried out at a plate temperature of50° to 100° C.

Thus treated metal substrate is then treated with a chromate solutioncontaining hexa-valent chromium containing chromium compound having asolubility of 20 to 10⁻⁵ and aqueous silica and/or aqueous resin.

The said chromium compound and binder material have already beendescribed above. In this chromate solution, the content of said chromiumcompound should preferably be in a range of 10⁻¹ to 100 g/l as metalchromium. This is because, if it is less than 10⁻¹ Cr g/l, it is toothin for the object of this invention and in a usual application methodand amount, the intended object and effect of the invention cannot beattained therewith. On the other hand, if it is over the upper limit of100 g/l, the stability of the treating composition per se will getworse. The binder content is preferably selected in a range of 10⁻² to500 g/l (on solid basis), and the pH of the solution is determined in arange of 1 to 10, as already stated above.

The chromate solution is applied so as to give a dried film weight of0.05 to 5 g/m², the chromium coverage, in terms of metal chromium, being0.01 g to 1 g/m².

If desired, said chromium solution may further include water solublechromium compound in an amount of 10⁻¹ to 100 g/l as metal chromium, andreaction promotor as stated hereinbefore. By the adoption of thistreating method, a further improved anti-corrosive layer can beobtained.

Alternatively, an excellent anti-corrosive layer of this invention maybe prepared by the method wherein a metal substrate is contacted with achromate solution containing a water soluble chromium compound having asolubility of more than 20 and aqueous silica and/or aqueous resin, and,after drying the same, contacted with a chromate solution containinghexa-valent chromium containing compound having a solubility of 20 to10⁻⁵, and aqueous silica and/or aqueous resin, and subjected to drying.In this method, each chromate solution is formulated so as to containthe defined chromium compound in an amount corresponding to 10⁻¹ to 100g Cr/l and aqueous silica and/or aqueous resin in an amount of 10⁻² to500 g/l on solid basis.

The chromate solution used in the second stage is adjusted to pH 1 to 10and may be added with water soluble chromate compound and reactionpromotor, if desired.

The chromium coverage in each layer and application of the respectivechromate solution are almost identical with those of the methodpreviously mentioned.

By the adoption of this method, an excellent anti-corrosive layer can beprovided on a metal substrate.

Thus, the anti-corrosive layer may be composed of a single layer or oftwo separate sub-layers, and therefore, in the present invention, unlessotherwise stated, the term "anti-corrosive layer" should be understoodas including all of the abovementioned layers.

The present object of providing a composite laminate consistingessentially of Zn, Fe or Al base plate, an anti-corrosive layer, and apaint layer free from chromium or lead compound, and being excellent inadhesion of coating and corrosion resistance both in initial andsuccessive stages, can be fully attained with the disclosed techniques.

A coated metal plate can be used in various technical fields. Amongthem, the most important and attractive one is in the automobileindustry.

In industrial production of automobile bodies or the like, therespective body portions such as, for example, locker panel, door,fender and the like, are prepared from steel panel by pressing,fabricated by spot welding and thereafter, degreased, treated byphosphating process and finally subjected to an electro depositioncoating.

Therefore, these panels are required to be excellent in processability,spot-weldability, adhesion of electrodeposited coating and corrosionresistance.

For spot-weldability, it is necessary to give electrical conductivity tothe coating, and for this purpose, the so-called conductive pigment isincluded in the coating.

However, in the actual production line of such automobile bodies where alarge quantity of materials are to be handled at higher speed with theleast number of operators, continuous spot weldability is an importantproblem for the art to solve. Incidentally, continuous spot weldabilityis evaluated by the number of weldings made before the cleaning orexchange of electrode is necessitated.

In the technique wherein electrical conductivity is given to the coatingby the inclusion of conductive pigment therein (e.g. Japanese PatentPublication No. 44569/77, Japanese Patent Application Kokai No.138758/83, ibid 174582/83) or the technique wherein the conductivity ofthe coating is further increased by exerting ingenuity in terms of theemployed pigment (e.g. Japanese Patent Application Kokai No. 79138/76,Japanese Patent Publication No. 19706/83), it is indeed possible tocarry out a certain number of spot-weldings, but they cannot standcontinuous welding of 2000 spots and more, because no thoughtfulconsideration is given to contamination of electrode. Neverthless, inthe latest, automobile production line, it is often desired to carry outa continuous spot-welding of 2000 to 5000 spots and more. On the otherhand, in the pressing or deep draw processing of the coated material, itis undesired that the coating peels off in powdery form, adheres to thepress, thereby causing processing marks or increasing the frequency ofcleaning of the press, or the coating peels off from the substrate ordevelops cracks, thereby decreasing corrosion resistance on that part.

For the purpose of improving processability of the coating, there hasbeen known the addition of inorganic or organic lubricant to the coatingcomposition.

However, a weldable coating is often formulated rich in pigment forattainment of electrical conductivity of the coating, and when suchinorganic pigments as molybdenum disulfide, carbon or lead oxide arecompounded with the coating composition in enough quantity to exert fullfunction thereof, additional problems of lowering the strength of thecoating and lowering processability necessarily occur.

In general, with a coating composition containing relatively largerquantity of conductive pigment or lubricant, there is a trend thatcorrosion resistance of the coating is lowered. Since some conductivepigments, by themselves or after being decomposed, may exert corrosionpromoting effects, careful selection of such pigments and theirconditions of use are, in fact, very important for the obtainment ofhighly anti-corrosive coating. Thus, in the related field, there haslong been desired to have a coated metal laminate having excellentprocessability, corrosion resistance, and continuous spot-weldability.

The coated metal plate should preferably have additional chracteristicsof being excellent in adhesion of finish coating like electro-depositioncoating, and corrosion resistance even at the imperfectly coated area.

The present inventors, having these considerations in mind, havediligently studied the correlation between the continuousspot-weldability and composition of the coating and have foud thatthermal decomposition of the binder resin in the coating is greatlyconcerned in the continuous spot-weldability, said thermal decompositionof the binder resin is greatly influenced by the aromatic ring contentof the resinous molecule and by the presence or absence of easilydecomposable material in the resin, and continuous spot-weldability canbe markedly improved by limiting the aromatic ring content of the binderresin in a certain range and compounding an appropriate amount ofthermally decomposable material in the coating composition.

As to the improvement in processability of the coated panel, theinventors have found that addition of organic lubricant can give a goodsolution to this problem and that such organic lubricant may alsofunction as the abovesaid thermally decomposable material. The requiredanti-corrosive property and adhesion of coating might be successfullyattained by the adoption of anti-corrosive layer hereinbefore stated.

On the basis of these findings, the inventors have succeeded inproviding a coated metal panel which is excellent in weldability,processability and corrosion resistance and is especially useful for theproduction of automobile bodies and the like in a commercial line.

Thus, in this aspect of the invention, there is provided ananti-corrosive, coated metal panel consisting essentially of metalsubstrate with or without anti-corrosive chemical pre-treatment, ananti-corrosive layer of the composition comprising at least onehexa-valent chromium containing chromium compound having a solubility of20 to 10⁻⁵, binder material composed of aqueous silica and/or aqueousresin and optional water soluble chromium compound, and an electricallyconductive coating layer, which is characterized by the fact that saidcoating layer is formed by applying either one of the following:

(A) a composition comprising

10 to 35% by weight of binder resin whose aromatic ring content in themolecule is 0 to 50% by weight,

4 to 50% by weight of polyethylene wax, and

15 to 86% by weight of electrically conductive pigment, or

(B) a composition comprising

10 to 35% by weight of binder resin whose aromatic ring content in themolecule is 0 to 50% by weight,

4 to 30% by weight of polyethylene wax,

15 to 85% by weight of electrically conductive pigment essentially ofdi-iron phosphide,

1 to 71% by weight of iron phosphide decomposition inhibitor, and 0 to70% by weight of other inorganic pigment, provided that the totalquantity of electrically conductive pigment, iron phosphidedecomposition inhibitor and other inorganic pigment is 35 to 86% byweight.

Thus, in the first instance, the coating layer is fomed by a coatingcomposition comprising, as solid, 10 to 35% by weight of at least onebinder resin having zero or less than 50% by weight of aromatic ringcontent, 4 to 50% by weight of polyethylene wax, and 15 to 98% by weightof electrically conductive pigment. The resinous component used in theabovesaid coating composition may be any binder resin whose aromaticring content in the molecule is 0 or 50% by weight at the most.

Examples of such resin are epoxy resin, alkyd resin, acryl resin,melamine resin, urethane resin, phenol resin, vinyl resin, polyvinylbutyral resin, polyvinyl acetate resin, chlorinated rubber, oil-freepolyester resin, phthalic acid resin, styrene resin, polyolefin resinand the like. They may be used either in organic solvent type or inaqueous type.

They are used alone or in combination form, and however, even in thelatter case, the aromatic ring content should be, as expressed in termsof average content, less than 50% by weight and most preferably 0%.

The inventors have found that the aromatic ring content of the resinousmolecule has a close connection with the aforesaid continuous spotweldability and when the aromatic ring content exceeds 50% by weight,the hereinunder defined thermal decomposition rate is extremely low:##EQU1## heating condition: 5 to 10 mg of sample resin is heated fromroom temperature to 550° C. at a rate of 20° C./ min in N₂ atmosphere;and

resinous weight after heating: when the sample resin reaches 550° C.,the resinous weight is determined by thermogravimetric analysis.

Further, continuous spot-weldability will also become extremely low inproportion therewith. However, when the aromatic ring content is 0 to50% by weight, the weight decreasing rate by thermal decomposition ofresin is in the order of 70 to 100%, and such material can well standcontinuous spot-welding of up to 5000 spots.

The resinous content in the coating composition is usually determined ina range of 10 to 35% by weight and prefereably 15 to 25% by weight.

If the resinous content is below the limit of 10% by weight, there is atrend that processability will become markedly decreased, and if theresinous content exceeds 35% by weight, there is a lowering ofcontinuous spot-weldability.

The polyethylene wax used in the present coating composition is in theform of powder having a mean diameter of less than 50 microns. Thepolyethylene should preferably have a molecular weight of 1,000 to10,000. Preferred examples are Ceridust 3620 (finely powderedpolyethylene wax, specific gravity of at least 0.94, acid value of notmore than 15 KOH mg/g, trade mark of Hoechst AG) and the like. Thepolyethylene wax is usually added in an amount corresponding to 4 to 50%by weight, preferably 4 to 30% by weight and most preferably 4 to 20% byweight of the total solids of the coating composition.

If the said amount is lower than 4% by weight, there is a decrease inprocessability and no thermal decomposition promoting activity towardthe resin, whereas if it exceeds 30% by weight, there is no furtherimprovememt in processability but rather decrease in corrosionresistance of the coating. However, said decrease in corrosionresistance is to a tolerable extent at the polyethylene wax levels of 30to 50% by weight. In the abovesaid coating composition, there is aninorganic pigment known as electrically conductive pigment.

Examples of such inorganic pigments are metal powders such as stainlesssteel, gold, silver, cadmium, aluminium, zinc, tin, copper, andgraphite-powders; metal carbides such as iron, nickel, titanium,tungsten, niobium and manganese carbides; and ferro alloys such asferromanganese, ferromolybdenum, ferrosilicon, ferrochromium,ferrotitanium, ferrophosphor and the like. They are used in pulverizedform with a mean diameter of less than 20μ, and preferably less than10μ.

The inorganic pigments are never limited to the above and any of theelectrically conductive pigments known in the art may be satisfactorilyused providing they give the least affect on corrosion resistance andprocessability of the coated plate. For example, in the combination ofsteel plate and metallic powder, it is preferred to exclude suchmetallic powder as having noble electrode potential as compared withiron electrode potential. Also, it is prefered to use such electricallyconductive pigment as having a low, stable electric resistance, notbeing fused at the time of welding, thereby showing no decrease inelectric current density and no contamination of electrode at that time,and having enough hardness to resist crushing under welding pressure.

Since the pigment is used in comparatively larger quantity, it is ofcourse desired to be supplied at a lower price. From this stand point,especially advantageous pigments are iron carbide and iron phosphide.

The electrically conductive pigment is usually employed in an amount of15 to 86% by weight, preferably 30 to 70% by weight, of the total solidof the coating composition. If the pigment amount is below the lowerlimit of 30% by weight, there are cases showing shortage of the desiredelectric conductivity at the film thickness of more than 10μ, and whenthe pigment amount is lower than 15% by weight, it is unable to carryout spot-weldings because of the clear shortage of electric conductivityof the coating. Whereas, when the pigment amount is over 70% by weight,there are cases wherein the processaibility will get worse, and at thelevel of over 86% by weight, there is a clear deterioration ofprocessability of the coated product.

In the present composition, other inorganic pigments such as, forexample, chromium pigments (zinc chromate, lead chromate, calciumchromate, strontium chromate pigments), phosphate pigments (zincphosphate, iron phosphate, aluminium tripolyphosphate pigments) and leadcompound pigments (lead silicate and the like) for the purpose ofincreasing corrosion resistance; body pigments (silica, calciumcarbonate, talc, alumina and the like) for the purpose of adjustingresinous content or coating pH; and coloring pigments (chromium oxide,iron oxide, lead oxide and the like) may be added if desired. This typeof coating composition may be prepared by blending the abovesaidcomponents in a usual way. That is, the resin is first dissolved in asolvent. To a part of said resinous solution, are added electricallyconductive pigment and other optional inorganic pigment and the mixtureis dispersed well by using triple roll mill, sand grind mill, ball mill,disolver and other known coating use dispersing means. To this, areadded the remaining amount of resinous solution and polyethylene wax andthe mixture is uniformly mixed and if desired, subjected to dispersingoperation again to obtain a coating composition. Though the solidcontent may vary with the coating method used, it is generally in arange of 10 to 70% by weight. Of course, other techniques may be usedfor the preparation of such coating composition and other additives maybe added if desired.

In the preparation of said coating composition, any of the conventionalcoating use solvents may be employed as, for example, xylene, toluene,mineral spirits, methanol, ethanol, butanol, isopropanol, methyl ethylketone, ethyl butyl ketone, methyl isobutyl ketone, ethyleneglycolmonobutyl ether, ethyleneglycol monoethyl ether, ethyleneglycolmonoethyl acetate, ethyl acetate, butyl acetate, cyclohexanone, waterand the like. The abovesaid coating composition may be applied by usinga conventional means on various metallic materials such as cold-rolledsteel plate, galvanized steel, zinc alloy plated sheet steel, aluminiumplated steel, zinc and zinc alloy sheet, aluminium and aluminium alloysheet, stainless steel, and various other steel plates. When themetallic substrate is contaminated with oil and other materials, it isfirst degreased by washing with an alkali-degreasing agent or organicsolvent. If desired, thus treated substrate is washed with water andthen subjected to a conventional chemical treatment to form phosphatecoating or chromate coating thereupon, washed and dried. Thus treatedsubstrate material is then applied with the present anti-corrosive layerand then with the abovesaid coating to obtain an anti-corrosive, coatedmetal plate, which is excellent in processability, corrosion resistanceand continuous spot-weldability. The said coating layer is usuallyapplied in 1 to 20 microns, preferably 2 to 10 microns, dry thickness,and the baking conditions are usually set at 100° to 300° plate maximumtemperature and 30 sec. to 2 min. baking time.

In the second instance, the coating layer is formed by using a coatingcomposition comprising, as solid, 10 to 35% by weight of binder resinwhose aromatic ring content is 0 to 50% by weight, 4 to 30% by weight ofpolyethylene wax, 15 to 85% by weight of electrically conductive pigmentessentially of di-iron phosphide, 1 to 71% by weight of iron phosphidedecomposition inhibitor, and 0 to 70% by weight of other inorganicpigment, provided that the total quantity of said electricallyconductive pigment, iron phosphide decomposition inhibitor and otheroptional pigment is 35 to 86% by weight.

In this composition, as the resinous component and the polyethylene wax,the same members as stated in connection with the aforesaid firstinstance are used in the indicated amounts. The electrically conductivepigment to be added for the purpose of giving electric conductivity tothe coating is specifically limited to the pigment composed essentiallyof iron phosphide (Fe₂ P) with a mean diameter of less than 20 micronsand most preferably less than 10 microns. This type of electricallyconductive pigment is prominent among others because of having a low,stable electric resistance, not being fused at the time of welding,thereby showing no decrease in electric current density and nocontamination of the electrode, having the desired hardness resist tocrushing, being inexpensive and supplied in larger quantity, and beinginert and excellent in anti-corrosive property. Various products aremarketed under the names of Ferrofos and the like and they are usedalone or in combination form. Though the selected pigments are the bestmaterials among the known electrically conductive pigments, it has beenfound that under specific corrosive circumstances such as an acidicatmosphere, iron phosphide may be decomposed and corrosion will bepromoted by the said decomposed substances.

It depends on the coating composition, substrate metal and corrosiveenvironmental conditions to determine whether the coating is broughtunder an acidic atmosphere or not. However, nothing could surpass acomposition that would give excellent corrosion resistance under severetests such as composite corrosion tests with several corrosiveenvironments, or alternate wet and dry conditions. The inventors havefound that in order to obtain such composition, it is essential tocontrol the decomposition of iron phosphide or to make the decomposedproducts harmless, and for this purpose, employment of certain type ofpigment whose aqueous suspension will show pH 6 to 13, preferably 7 to11, can provide a very effective solution, because it is effective tomaintain the coating always under neutral to alkaline condition.

Therefore, in this aspect of the invention, the coating compositioncontains as essential ingredients, iron phosphide decompositioninhibitor, together with an electrically conductive pigment composedessentially of iron phosphide.

As already stated, the only role of this inhibitor is to bring thecoating always under neutral or alkaline condition and any pigment whoseaqueous suspension shows pH 6 to 13 may be satisfactorily used for thisend. Examples of such pigment are:

(1) alkaline earth metal petroleum sulfonate,

(2) chromate pigments such as zinc chromate, zinc potassium chromate,lead chromate, basic lead chromate, calcium chromate and strontiumchromate,

(3) phosphate pigments such as zinc phosphate, and iron phosphate,aluminium tripolyphosphate,

(4) lead compound pigments such as lead silicate,

(5) body pigments such as talc, calcium carbonate and silica, and

(6) zinc powder (less than the iron phosphide amount).

Incidentally, the amount of this type of pigment should preferably bemore than 1% by weight of the total solid of the coating composition,but less than the hereinafter stated total amount of inorganic pigments.

They are sometimes classified as anti-corrosive pigment or body pigment,but in this invention, are recognized as iron phosphide decompositioninhibitor because of having the defined aqueous suspension pH.

In this type of coating composition, other inorganic pigments for thepurpose of improving corrosion resistance, control of resinous content,pH control, coloring and the like may be added if desired. As to thedetails of these pigments, reference should be made to the precedingcoating composition.

However, in this aspect of the invention, the total quantity ofelectrically conductive pigment composed essentially of iron phosphide,iron phosphide decomposition inhibitor and other optional inorganicpigment should be in a range of 35 to 86% by weight, preferably 35 to70% by weight of the coating solid. This is because, if the total amountof said pigments is less than 35% by weight, there is a decrease inprocessability and even at the level of more than 70% by weight, thereis the case wherein the processability will be somewhat decreased. Whenit exceeds upper limit of 86% by weight, there is a marked decrease inprocessability.

In formulating the coating composition, the selected resin is firstdissolved in a solvent. To a part of said resinous solution, are addedelectrically conductive pigment composed essentially of di-ironphosphide decomposition inhibitor and other optional inorganic pigmentif any, and the mixture is dispersed well. Next, the remaining amount ofthe resinous solution and polyethylene wax are added and mixed well toobtain a coating composition. The solid content may be controlled to 10to 70% by weight, though it may vary in wide range depending on thecoating method, coating object or the like. To this composition, otherconventional additives may be added if desired. By the use of thiscoating composition, together with the present anti-corrosive layer, itis possible to obtain a very attractive coated metal plate havingimproved processability, anti-corrosive property and particularlyexcellent continuous spot-weldability.

The invention shall be now more fully explained in the followingExamples. Unless otherwise stated, all parts and percentages are byweight.

EXAMPLE 1

(1) metal substrate:

Hot dip alloy-galvanized sheet steel (60/60 oiling) was treated withalkali-degreasing agent (Ridoline 75, manufactured by Nippon Paint Co.)

(2) pre-treating liquids:

    ______________________________________                                        1st stage liquid (reaction type chromate treating agent)                      chromium trioxide                                                                              12 g/l  (in terms of                                                                  metallic chromium)                                   phosphoric acid  10 g/l  (in terms of                                                                  non-volatile content)                                water                                                                         2nd stage liq. (chromate liq.)                                                zinc potassium chromate                                                                        15 g/l  (in terms of                                         (water solubility 6 to 8)                                                                              metallic chromium)                                   Snowtex N        15 g/l  (in terms of                                                                  non-volatile content)                                water                                                                         ______________________________________                                    

Each liquid was prepared by mixing the indicated components andsubjecting the same to a glass bead dispersion in a paint shaker for 1hour.

(3) pre-treatment

The degreased metal substrate was dipped in the abovesaid 1st stageliquid at 50° C. for 10 sec., dried in hot air, coated with the 2ndstage liquid by a bar coater (#3) at room temperature, and withoutwashing, dried at 100° C. (maximum plate temperature 100° C.) for 1 min.

(4) paint

Urethane modified epoxy resin base clear paint (solvent type).

(5) coating method

The abovesaid pre-treated steel plate was coated with the said paint bya bar coater and baked at 200° C. for 1 min. (maximum plate temperature200° C.). The dry thickness of the coating was 2 μ.

EXAMPLE 2

Similar experiment was carried out as in Example 1 excepting using thechromate liquids as shown in Table 1.

EXAMPLE 3

(1) metal substrate

Electroplated galvanized sheet iron was degreased with analkali-degreasing agent Ridoline 155 (manufactured by Nippon Paint Co.,Ltd.).

(2) pre-testing liquid (reaction type chromate solution)

Granodine 92 (Nippon Paint Co., Ltd.) added with hydrochloric acid(reaction promotor) was used.

(3) pre-treatment

The degreased metal substrate was dipped in 3% (v/v) aqueous Granodine92 (Nippon Paint Co., Ltd.) solution at 40° C. for 10 sec., washed withwater, washed with hot water, and dried in hot air. Thus treatedsubstrate material was then treated with the 2nd stage chromate liquidshown in Table 1 as in Example 1.

EXAMPLE 4

Electroplated galvanized sheet iron was spray-treated with a degreasing,reaction type chromate treating agent of the following composition at50° C. for 10 sec., washed with water, water drained, dried and treatedthereafter as in Example 2.

Reaction type chromate treating liquid:

    ______________________________________                                        30% reduced chromium trioxide*                                                                     7 g/l (as Cr)                                            sulfuric acid        5 g/l (as non-volatile)                                  polyacrylic acid     1 g/l (as non-volatile)                                  Adekanol LO-7      0.5 g/l (as non-volatile)                                  Adekanol B 2020    0.5 g/l (as non-volatile)                                  (nonionic surfactant,                                                         manufactured by                                                               Asahi denka K.K.)                                                             ______________________________________                                         *aq. CrO.sub.3 solution was reduced by formalin                          

COMPARATIVE EXAMPLES 1 TO 2

The comparative samples were prepared as in Examples by using theprescriptions given in Table 1.

                                      TABLE 1                                     __________________________________________________________________________    Example     1       2       3                                                 __________________________________________________________________________    Substrate   hot dip alloy                                                                         hot dip alloy                                                                         zinc                                              (plated steel)                                                                            galvanized                                                                            galvanized                                                                            electroplated                                     1st stage chromate                                                                        chromium                                                                              CrO.sub.3                                                                             Granodine 92                                                  trioxide CrO.sub.3                                                chromium compound                                                                         Cr 12 g/l                                                                             Cr 7 g/l                                                  reaction promoter                                                                         phosphoric                                                                            potassium                                                                             hydrochloric                                                  acid 10 g/l                                                                           fluotitanate                                                                          acid                                              washing with water                                                                        no      no      yes                                               drying      PMT 50° C.                                                                     PMT 50° C.                                                                     PMT 50° C.                                 total Cr (1st stage                                                                       50 mg/m.sup.2                                                                         30 mg/m.sup.2                                                                         100 mg/m.sup.2                                    Cr-coverage)                                                                  2nd stage chromate                                                                        ZPC     ZPC     ZPC                                               A.sub.1 chromium content                                                                  Cr 15 g/l                                                                             Cr 15 g/l                                                                             Cr 5 g/l                                          A.sub.2 chromium content*                                                                 --      I Cr.sup.6+ 90%                                                                       --                                                                    total Cr 20 g/l                                           binder**    II NV 15 g/l                                                                          III NV 15 g/l                                                                         II NV 15 g/l                                      drying      PMT 100° C. ×                                                            PMT 100° C. ×                                                            PMT 100° C. ×                                    1 min.  1 min.  1 min.                                            composite coat.                                                               total Cr    110 mg/m.sup.2                                                                        170 mg/m.sup.2                                                                        120 mg/m.sup.2                                    A.sub.1 chromium                                                                           60 mg/m.sup.2                                                                         60 mg/m.sup.2                                                                         20 mg/m.sup.2                                    painting*** V 2μ PMT                                                                           V 2μ PMT                                                                           V 2μ PMT                                                   200° C. 1 min.                                                                 200° C. 1 min.                                                                 200° C. 1 min.                             salt spray test                                                               initial**** 1200 H  1500 H  1500 H                                            successive****                                                                            1200 H  1500 H  1500 H                                            __________________________________________________________________________    Example     Comp. Ex. 1                                                                           Comp. Ex. 2                                                                           4                                                 __________________________________________________________________________    Substrate   hot dip alloy                                                                         hot dip alloy                                                                         zinc                                              (plated steel)                                                                            galvanized                                                                            galvanized                                                                            electroplated                                     1st stage chromate                                                                        CrO.sub.3                                                                             CrO.sub.3                                                                             CrO.sub.3                                         chromium compound                                                                         Cr 20 g/l                                                                             Cr 20 g/l                                                                             30% red. 7 g/l                                    reaction promoter                                                                         phosphoric                                                                            phosphoric                                                                            surfactant/                                                   acid 10 g/l                                                                           acid 10 g/l                                                                           sulfuric acid                                     washing with water                                                                        no      no      yes                                               drying      PMT 50° C.                                                                     PMT 50° C.                                                                     PMT 50° C.                                 total Cr (1st stage                                                                       80 mg/m.sup.2                                                                         80 mg/m.sup.2                                                                         30 mg/m.sup.2                                     Cr-coverage)                                                                  2nd stage chromate          ZPC                                               A.sub.1 chromium content    Cr 15 g/l                                         A.sub.2 chromium content*                                                                         I Cr 20 g/l                                                                           Cr.sup.6+ 70%                                                                 total cr 20 g/l                                   binder**            III NV 15 g/l                                                                         III NV 15 g/l                                     drying              PMT 100° C. ×                                                            PMT 100° C. ×                                            1 min.  1 min.                                            composite coat.                                                               total Cr    80 mg/m.sup.2                                                                         160 mg/m.sup.2                                                                        170 mg/m.sup.2                                    A.sub.1 chromium             60 mg/m.sup.2                                    painting*** V 2μ PMT                                                                           V 2μ PMT                                                                           V 2μ PMT                                                   200° C. 1 min.                                                                 200° C. 1 min.                                                                 200° C. 1 min.                             salt spray test                                                               initial****  200 H  1000 H  1200 H                                            successive****                                                                             72 H    200 H  1200 H                                            __________________________________________________________________________     *I chromium trioxide was partially reduced with formalin                      **II Snowtex N; III Aerosil 300                                               ***V urethane modified epoxy resin clear                                      ****generation of white rust                                                  PMT-- peak metal temperature                                                  ZPC-- zinc potassium chromate                                            

EXAMPLE 5

Electroplated galvanized sheet iron was coated by a bar coater with the1st stage chromate liquid (a₁) shown in Table 2 in an amount of 4 ml/m²,and without washing, subjected to drying (maximum plate temperature 100°C.) for 1 minute.

Next, the 2nd stage chromate liquid (b₁) was applied as in the firststage coating, and dried.

Thereafter, epoxy base clear coating was coated in dry thickness of 2μ.

The anti-corrosive property was evaluated by using salt spray method andcounting the time up to white rust generation. The test was conducted atthe time of just after the coating and after being kept at roomtemperature for 2 months. The test results are shown in Table 2.

EXAMPLE 6 AND COMPARATIVE EXAMPLE 3 AND 4

Similar coated plates were prepared using the method of Ex. 5 butsubstituting the chromate liquids shown in Table 2 and 3 for that of Ex.5.

Incidentally, in the following Tables 2 and 3, the following were usedas chromium compounds: zinc potassium chromate having a solubility of 6to 8 and strontium chromate having a solubility of 10⁻¹.

                                      TABLE 2                                     __________________________________________________________________________    chromate solutions                                                                   a.sub.1 a.sub.2 a.sub.3  b.sub.1                                                                             b.sub.2                                 __________________________________________________________________________    A.sub.1 chromium                                                                     no      no      no       strontium                                                                           zinc potassium                                                          chromate                                                                            chromate (ZPC)                          Cr concent.                     15 g/l                                                                              3 g/l                                   A.sub.2 chromium                                                                     chromium tri-                                                                         CrO.sub.3                                                                             CrO.sub.3                                                                              no    CrO.sub.3                                      oxide (CrO.sub.3)                                                                     formalin reduc.                                                                       formalin reduc.                                                                              formalin reduc.                         Cr concent.                                                                          formalin reduc.                                                                       Cr 40 g/l                                                                             Cr 20 g/l      Cr 10 g/l                               Cr.sup.6+ /                                                                          Cr 20 g/l                                                              total Cr                                                                             70%     70%     70%            70%                                     B-binder                                                                             Aerosil 300                                                                           Aerosil 300                                                                           Jurymer AC-10L                                                                         Snowtex N                                                                           Jurymer AC-10L                          as NV  15 g/l  30 g/l  10 g/l   15 g/l                                                                              7 g/l                                   pH     2       2       2        9     2                                       __________________________________________________________________________     Jurymer AC10L (polyacrylic acid, manufactured by Nihon Junyaku K.K.)          b liquid . . . aqueous dispersion was subjected to a glass bead dispersio     in paint shaker for 1 hour                                               

                  TABLE 3                                                         ______________________________________                                        Example    5       6       Comp. Ex. 3                                                                            Comp. Ex. 4                               ______________________________________                                        1st stage liquid                                                                         a.sub.1 a.sub.1 a.sub.2  a.sub.1                                   coating                                                                       A.sub.1 Cr mg/m.sup.2                                                                     0       0       0        0                                        A.sub.2 Cr mg/m.sup.2                                                                    80      80      160      80                                        2nd stage liquid                                                                         b.sub.1 b.sub.2 no       a.sub.3                                   Composit coating                                                              total Cr mg/m.sup.2                                                                      140     132     160      160                                       A.sub.1 -Cr mg/m.sup.2                                                                   60      12       0        0                                        Evaluation                                                                    initial    1000 H  800 H   700 H    700 H                                     anticorrosion                                                                 successive 1000 H  800 H   200 H    200 H                                     anticorrosion                                                                 ______________________________________                                    

EXAMPLE 7

Zinc potassium chromate having a solubility of 6 to 8 (15 g as metallicchromium), polyacrylic acid Jurymer AC-10L (manufactured by NihonJunyaku K. K., 10 g as solid) and water (to make the total volume to1 1) were compounded and subjected to a glass bead dispersion in a paintshaker for 1 hour, and adjusted to pH 2 to obtain a metal coatingpre-treating agent.

EXAMPLES 8 TO 17 AND COMPARATIVE EXAMPLES 5 TO 6

Using the method of Example 7, but following the prescriptions given inthe Table 4, the respective pre-treating agents were obtained.

For the adjustment of pH to an acidic side, acetic acid was used and toan alkali side, ammonia water.

In Example 15, pH was adjusted to 9 and in other Examples andComparative Examples adjusted to 2.

In Example 16, 7 g/l of phosphoric acid was added as a reactionpromotor.

The abovementioned treating agents were each applied by roll-coattechnique to electroplated galvanized steel sheet (Examples 7 to 16 andComparative Examples 5 to 6) or hot dip alloy-galvanized sheet iron(Example 17) previously degreased with an alkali degreasing agent(Ridoline 155, manufactured by Nippon Paint Co., Ltd.), dried, appliedwith a clear paint in a dry thickness of 2μ, and dried by applying heat.The coating amounts of treating agents on the steel sheets were eachcontrolled to 4 ml/m². With these coated plates, chromium flow-out andanti-corrosive properties were examined.

In these examples, solvent type urethane modified epoxy resin clear wasused as paint, but in Example 17, an aqueous carboxylated polyethyleneresin clear paint was used. Anti-corrosive property was evaluated byusing salt-spray method and measuring the time until white rust willcome out with the sample just prepared and the sample maintained at roomtemperature for two months. The test results are shown in Table 4.

The chromium flow-out was evaluated by using the sample whose back andedge portions were sealed, dipping it in a pure water at 40° C. for 72hours, measuring the chromium amount adhered on the surface byfluorescent X-ray analysis, and calculating the chromium flow-out. Innone of the plates of Examples 7 to 17 and Comparative Examples 5 to 6,was chromium flow-out detected. In these Examples and ComparativeExamples, the following were used:

zinc potassium chromate having a solubility of 6 to 8,

calcium chromate having a solubility of 14,

lead chromate having a solubility of 10⁻⁴, and

sodium chromate having a solubility of 64.

                                      TABLE 4                                     __________________________________________________________________________    metal coating pretreating agents                                              __________________________________________________________________________    Example 7         8         Comparative 5                                                                           9                                       __________________________________________________________________________    chromate liq.                                                                         zinc potassium                                                                          ZPC       no        calcium chromate                        A.sub.1 chromium                                                                      chromate (ZPC)                                                        Cr conc.                                                                              Cr 15 g/l Cr 15 g/l           Cr 15 g/l                               A.sub.2 chromium                                                                      no        chromium trioxide                                                                       chromium trioxide                                                                       chromium trioxide                                         formalin reduction                                                                      formalin reduction                                                                      formalin reduction                      Cr.sup.6+ /       Cr 20 g/l Cr 30 g/l Cr 20 g/l                               total Cr          70%       70%       70%                                     B-binder                                                                              polyacrylic acid                                                                        Aerosil 300                                                                             Aerosil 300                                                                             Aerosil 300                                     Jurymer AC-10L                                                        as NV   10 g/l    15 g/l    23 g/l    15 g/l                                  Evaluation                                                                    anticorrosion                                                                 initial 500 H     800 H     700 H     700 H                                   successive                                                                            400 H     800 H     200 H     700 H                                   __________________________________________________________________________    Example 10        11        Comparative 6                                                                           12                                      __________________________________________________________________________    chromate liq.                                                                         lead chromate                                                                           calcium chromate                                                                        sodium chromate                                                                         zinc potassium                          A.sub.1 chromium  5 g/l               chromate (ZPC)                                            ZPC 5 g/l                                                                     lead chromate 5 g/l                                         Cr conc.                                                                              Cr 15 g/l Cr 15 g/l Cr 15 g/l Cr 15 g/l                               A.sub.2 chromium                                                                      CrO.sub.3 CrO.sub.3 CrO.sub.3 CrO.sub.3                                       formalin reduction                                                                      formalin reduction                                                                      formalin reduction                                                                      formalin reduction                      Cr.sup.6+ /                                                                           Cr 20 g/l Cr 20 g/l Cr 20 g/l Cr 20 g/l                               total Cr                                                                              70%       70%       70%       70%                                     B-binder                                                                              Aerosil 300                                                                             Aerosil 300                                                                             Aerosil 300                                                                             Snowtex 0                               as NV   15 g/l    15 g/l    15 g/l    15 g/l                                  Evaluation                                                                    anticorrosion                                                                 initial 600 H     1000 H    500 H     800 H                                   successive                                                                            600 H     1000 H    200 H     800 H                                   __________________________________________________________________________    Example 13    14       15      16      17                                     __________________________________________________________________________    chromate liq.                                                                         ZPC   ZPC      ZPC     ZPC     ZPC                                    A.sub.1 chromium                                                              Cr conc.                                                                              Cr 15 g/l                                                                           Cr 15 g/l                                                                              Cr 15 g/l                                                                             Cr 15 g/l                                                                             Cr 15 g/l                              A.sub.2 chromium                                                                      CrO.sub.3                                                                           chromium nitrate                                                                       Na bichromate                                                                         CrO.sub.3                                                                             CrO.sub.3                                                             formalin reduct.                                                                      formalin reduct.                       Cr.sup.6+ /                                                                           Cr 20 g/l                                                                           Cr 20 g/l                                                                              Cr 20 g/l                                                                             Cr 20 g/l                                                                             Cr 20 g/l                              total Cr                                                                              100%  0%       100%    70%     70%                                    B-binder                                                                              Aerosil 300                                                                         Aerosil 300                                                                            Snowtex N                                                                             Aerosil 300                                                                           Aerosil 300                            as NV   15 g/l                                                                              15 g/l   15 g/l  15 g/l  15 g/l                                 Evaluation                                                                    anticorrosion                                                                 initial 600 H 600 H    600 H   1000 H  1200 H                                 successive                                                                            600 H 600 H    600 H   1000 H  1200 H                                 __________________________________________________________________________

EXAMPLE 18

Zinc potassium chromate having a solubility of 6 to 8 (15 g as metallicchromium), polyacrylic acid Jurymer AC-10L (10 g as solid) and water (tomake the total volume to 1 1) were compounded and subjected to a glassbead dispersion with a paint shaker for 1 hour.

Thus, a metal coating pre-treating agent (pH 2 to 3) was obtained.

EXAMPLES 19 TO 21 AND COMPARATIVE EXAMPLES 7 TO 9

Using the same method as stated in Example 18, but following theprescriptions given in Table 5, the respective pre-treating agents wereobtained.

For the purpose of adjustment of pH to an acidic side, acetic acid wasused and to an alkali side, an ammonia water. The respectivepre-treating agents were applied by roll-coat technique ontoelectroplated galvanized steel sheet, dried, applied with a clear paintin a dry thickness of 2μ, and dried under heating.

The chromium flow-out and anti-corrosive properties were examined.

The following standards were used for the evaluation of chromiumflow-out.

○ : now flowing out

Δ: flow-out to the extent of less than 5 mg/m²

×: flow-out to the extent of more than 5 mg/m²

                                      TABLE 5                                     __________________________________________________________________________    Example 18    19    20    Comp. 7                                                                             Comp. 8                                                                             Comp. 9                                                                             21                                __________________________________________________________________________    metal subst.                                                                          zinc electroplated steel sheet                                        metal coating pre-treating agent                                              A.sub.1 chromium                                                                      I (15)                                                                              II (5)                                                                              II (1)                                                                              --    --    --    I (15)                            compound      I  (5)                                                                              I  (1)                                                    (Cr g/l)      III (5)                                                                             III (1)                                                   A.sub.2 chromium                                                                      --    IV (20)                                                                             IV (20)                                                                             IV (40)                                                                             IV (40)                                                                             IV (40)                                 compound                                                                      (Cr g/l)                                                                      Cr.sup.6+ /total Cr                                                                   --     70%   70%   70%   70%   70%  --                                binder  V     VI    VI    VI    VI    VI    VII                               NV g/l  10    15    15    30    30    30    15                                pH      2˜3                                                                           2˜3                                                                           2˜3                                                                           2˜3                                                                           2˜3                                                                           2˜3                                                                            9                                anti-corrosive layer                                                          total Cr                                                                              60 mg/m.sup.2                                                                       140 mg/m.sup.2                                                                      92 mg/m.sup.2                                                                       160 mg/m.sup.2                                                                      160 mg/m.sup.2                                                                      160 mg/m.sup.2                                                                      60 mg/m.sup.2                     A.sub.1 60 mg/m.sup.2                                                                        60 mg/m.sup.2                                                                      12 mg/m.sup.2                                                                        0     0      0   60 mg/m.sup.2                     paint   VIII  VIII  VIII  VIII  IX    IX    X                                 thichness                                                                             2μ 2μ 2μ 2μ 2μ 2μ 1μ                             Cr flow-out                                                                           ○                                                                            ○                                                                            ○                                                                            ○                                                                            Δ                                                                             X     ○                          initial SST                                                                           500 H 1000 H                                                                              800 H 700 H 700 H 1000 H                                                                              500 H                             successive                                                                            500 H 1000 H                                                                              800 H 200 H 200 H  600 H                                                                              500 H                             SST                                                                           __________________________________________________________________________     anti-SST time up to white rust generation                                     I. zinc potassium chromate (solubility 6˜8)                             II. calcium chromate (solubility 14)                                          III. lead chromate (solubility 10.sup.-4)                                     IV. chromium trioxide (partial reduction)                                     V. polyacrylic acid Jurymer AC10L                                             VI. aq. dispersible silica Aerosil 300                                        VII. Snowtex N                                                                VIII. urethane modf. epoxy resin clear                                        IX. urethane modif. epoxy containing strontium chromate Contents: Comp. 8     12 mg/m.sup.2 (as Cr); Comp. 9: 100 mg/m.sup.2 (as Cr)                        X. carboxylated polyethylene clear                                       

EXAMPLE 22

20 parts of resin (A), which was a copolymer of isobutyl methacrylateand styrene having a weight average molecular weight of 200,000 andcontaining 10% by weight of benzene rings were dissolved in 66 parts ofcyclohexanone. To this, were added 60 parts of Ferrophos HRS 2132(electrically conductive pigment essentially of di-iron phosphide,manufactured by Hooker Chemicals and Plastics Corp.) and the mixture wassubjected to steel bead dispersion in paint shaker for 2 hours.

15 parts of Ceridust 3620 (finely powdered polyethylene wax,manufactured by Hoechst AG.) were added and dispersed well to obtain acoating composition having a non-volatile content of about 60% byweight.

EXAMPLES 23 TO 30 AND COMPARATIVE EXAMPLES 10 TO 13

Using the same procedure as stated in Example 22 but varying the kindsand amounts of resin, electrically conductive pigment, other inorganicpigment and lubricant as shown in Table 6, various coating compositionswere prepared.

                                      TABLE 6                                     __________________________________________________________________________    Example                                                                              23    24    25    Comp. 10                                                                            Comp. 11   Comp. 12                            __________________________________________________________________________    resin  B + C B + C B + C D     B + C      B + C                               wt ratio                                                                             B/C = 8/2                                                                           B/C = 8/2                                                                           B/C = 8/2   B/C = 8/2  B/C = 8/2                           benzene ring                                                                          47%   47%   47%   54%   47%        47%                                content                                                                       amount %                                                                             20    12    30    20    40         30                                  elect. cond.                                                                         Ferrophos                                                                           Ferrophos                                                                           Ferrophos                                                                           Ferrophos                                                                           Ferrophos  Ferrophos                           pigment                                                                              HRS 2132                                                                            HRS 2132                                                                            HRS 2132                                                                            HRS 2132                                                                            HRS 2132   HRS 2132                            amount %                                                                             60    65    50    60    40         65                                  polyethylene                                                                         Ceridust                                                                            Ceridust                                                                            Ceridust                                                                            Ceridust                                                                            Ceridust   Ceridust                            wax    3620  3620  3620  3620  3620       3620                                amount %                                                                             15    18    15    15    15          0                                  __________________________________________________________________________    Example                                                                              26    27    28    Comp. 13                                                                            29         30                                  __________________________________________________________________________    resin  B + C B + C B + C B + C B + C      B + C                               wt ratio                                                                             B/C = 8/2                                                                           B/C = 8/2                                                                           B/C = 8/2                                                                           B/C = 8/2                                                                           B/C = 8/2  B/C = 8/2                           benzene ring                                                                          47%   47%   47%   47%   47%        47%                                content                                                                       amount %                                                                             20    20    20    60    20         20                                  elect. cond.                                                                         Ferrophos                                                                           Ferrophos                                                                           Ferrophos                                                                           Ferrophos                                                                           Ferrophos  Ferrophos                           pigment                                                                              HRS 2132                                                                            HRS 2132                                                                            HRS 2132                                                                            HRS 2132                                                                            2132 (30 wt %)                                                                           2132 (30 wt %)                                                     Zn powd (30 wt %)                                                                        Zn powd. (30 wt %)                  amount %                                                                             50    65    30    25    60         60                                  polyethylene                                                                         Ceridust                                                                            Ceridust                                                                            Ceridust                                                                            Ceridust                                                                            Ceridust   butyl                               wax    3620  3620  3620  3620  3620       stearate                            amount %                                                                             25    15    15    15    15         15                                  __________________________________________________________________________

Incidentally, in the Table 6, the materials used were

resin B: urethane modified epoxy resin (Example 3 of Japanese patentapplication Kokai No. 30717/82), benzene ring content being 45 wt %

resin C: resol type phenol resin (Showa Union Gosei K.K., BKS-316),benzene ring content being 55 wt %

resin D: epoxy resin (Epo-tohto D-014, epoxy equivalent 950,manufactured by ToTo Kaseisha), benzene ring content being 54 wt %

Zinc Powder: manufactured by Mitsui Kinzoku Kogyo K. K., coating usezinc powder

Zinc nickel alloy electroplated steel plate (40 g/m²) was treated withalkali-degreasing agent, washed with water and dried, and then it wascoated with a chromate solution by a roll-coater so as to give chromiumadhered amount of 200 mg/m², and dried to obtain a pre-treated sheelplate. As the chromate solution, use was made of the solution preparedby reducing an aqueous chromium trioxide solution with formalin, therebyobtaining a solution containing 20 g/l of total chromium (Cr⁶⁺ / Cr³⁺=7/3), adding aerosil 300 (fumed silica, manufactured by Nihon AerosilCo.) in an amount of 15 g/l, and subjecting to a glass bead dispersionin a paint shaker for 1 hour.

To the abovesaid steel plate, the coating compositions (solid 60%)obtained in Examples 22 to 30 and Comparative Examples 10 to 13 wereeach applied by roll-coater, to give a dry thickness of 5μ, baked in adirect hot gas furnace at the maximum plate temperature of 200° C. for60 seconds and allowed to cool to obtain a test specimen. With thesetest specimens, the following tests were carried out.

Continuous spot-weldability:

Under the conditions of electrode diameter 5 mm, pressure 200 Kg,charged times 10 cycles, and electric current 8000A, spot-weldings werecarried out and the number of continuously welded spots was determined.

Evaluation:

⊚ : more than 5000 spots, no contamination of electrode

○ : more than 5000 spots, considerable contamination of electrode

Δ: 3000 to 5000 spots

×: less than 3000 spots

The test results are shown in Table 7.

                  TABLE 7                                                         ______________________________________                                                  Continuous spot-weldability                                         ______________________________________                                        Example                                                                       22          ⊚                                                  23          ⊚                                                  24          ⊚                                                  25          ○                                                          26          ⊚                                                  27          ⊚                                                  28          ⊚                                                  29          ⊚                                                  30          ⊚                                                  Comp. Ex.                                                                     10          X                                                                 11          X                                                                 12          X                                                                 13          X                                                                 ______________________________________                                         Satisfactory processability and anti-corrosive property were obtained with     each specimen prepared by using the compositions of Examples 22 to 30, but     poor processabilities were obtained with those of Comparative Examples 12     and 13.

EXAMPLE 31

Urethane modified epoxy resin B (manufactured by the method of Example 3of Japanese Patent Application Kokai No. 30717/82, benzene ring content45% by weight) and resol type phenol resin C (Showa Union Gosei K.K.,BKS 316, benzene ring content 55% by weight) were mixed in 8/2 ratio toobtain mixed resin (average benzene ring content 47% by weight).

20 parts of said mixed resin were dissolved in 66 parts of cyclohexanoneand to this solution, were added 60 parts of Ferrophos HRS 2132(electrically conductive pigment essentially of di-iron phosphide,manufactured by Hooker Chem. and Plastics Corp.) and 5 parts ofstrontium chromate N (chromate pigment, aqueous suspenson pH 8 to 9,manufactured by Kikuchi Shikiso K.K.) and the mixture was subjected to asteel bead dispersion in a paint shaker for 2 hours. Thereafter, 15parts of Ceridust 3620 (finely pulverized polyethylene wax, manufacturedby Hoechst AG.) were added and mixed uniformly by using Disper to obtaina coating composition having a non-volatile content of 65% by weight.

EXAMPLES 32 TO 36 AND COMPARATIVE EXAMPLES 14 TO 16

Using the same method as stated in Example 31, but changing the kindsand amounts of resin, electrically conductive pigment, iron phosphidedecomposition inhibitor and polyethylene wax as given in the followingTable 8, various coating compositions were obtained.

                                      TABLE 8                                     __________________________________________________________________________    Example    32      33      34      35                                         __________________________________________________________________________    resin      B + C   B + C   B + C   B + C                                      wt ratio   (B/C = 8/2)                                                                           (B/C = 8/2)                                                                           (B/C = 8/2)                                                                           (B/C = 8/2)                                average benzene                                                                          47      47      47      47                                         ring content (wt %)                                                           amount (wt %)                                                                            20      20      20      20                                         elec. conductive                                                                         Ferrophos HRS                                                                         Ferrophos HRS                                                                         Ferrophos HRS                                                                         Ferrophos HRS                              pigment    2132    2132    2132    2132                                       amount (wt %)                                                                            60      35      30      55                                         polyethylene wax                                                                         Ceridust 3620                                                                         Ceridust 3620                                                                         Ceridust 3620                                                                         Ceridust 3620                              amount (wt %)                                                                            15      15      15      15                                         decomp. inhibitor                                                                        SACI 700                                                                              Imsil A-108                                                                           strontium                                                                             strontium                                  pH of aq. suspens.                                                                       (8.5˜9.5)                                                                       (6.5˜7.5)                                                                       chromate N 5                                                                          chromate N 5                                                          Sicor ZmP/S                                                                           SACI 700                                   amount (wt %)                                                                             5      30      30       5                                         total pigment                                                                            65      65      65      65                                         (wt %)                                                                        __________________________________________________________________________    Example    36      Comp. Ex. 14                                                                          Comp. Ex. 15                                                                          Comp. Ex. 16                               __________________________________________________________________________    resin      B + C   B + C   B + C   B + C                                      wt ratio   (B/C = 8/2)                                                                           (B/C = 8/2)                                                                           (B/C = 8/2)                                                                           (B/C = 8/2)                                average benzene                                                                          47      47      47      47                                         ring content (wt %)                                                           amount (wt %)                                                                            20      20      20      20                                         elec. conductive                                                                         Ferrophos HRS                                                                         Zn Powder                                                                             Ferrophos HRS                                                                         Ferrophos HRS                              pigment    2132            2132    2132                                       amount (wt %)                                                                            30      65      65      60                                         polyethylene wax                                                                         Ceridust 3620                                                                         Ceridust 3620                                                                         Ceridust 3620                                                                         Ceridust 3620                              amount (wt %)                                                                            15      15      15      15                                         decomp. inhibitor                                                                        strontium               VULCAN                                     pH of aq. suspens.                                                                       chromate N 5            XC-72R                                                Zn powder               (4.5˜5.5)                            amount (wt %)                                                                            30                       5                                         total pigment                                                                            65      65      65      65                                         (wt %)                                                                        __________________________________________________________________________       Incidentally, in this Table 8, the material used were

Imsil A-108: amorphous soft silica, manufactured by Illinois MineralsCo.

Zinc powder: coating use zinc powder, manufactured by Mitsui KinzokuKogyo K.K.

SACI 700: mixture of calcium salt of petroleum sulfonate and calciumcarbonate, manufactured by Witco Co.

Sicor Z mp/S: zinc phosphate pigment, manufactured by BASF

Vulcan XC-72R: carbon black, manufactured by Cabot Co.

Zinc nickel alloy electroplated steel sheet (40 g/m²) wasalkali-degreased, washed with water, dried, and then coated by means ofa roll=coater with a chromate solution so as to give a chromium adheredamount of 200 mg/m², and dried to obtain a pre-treated steel sheet.

As the chromate solution, use was made of the solution obtained byreducing an aqueous chromium trioxide solution with formalin therebyforming the solution containing total chromium of 20 g/l consisting ofCr⁶⁺ /Cr³⁺ =7/3, adding aerosil 300 (fumed silica, manufactured by NihonAerosil Co.) in an amount of 15 g/l, and subjecting to a glass beadsuspension in a paint shaker for 1 hour. To thus prepared steel sheet,the coating compositions obtained in each Example and ComparativeExample were applied by roll-coater in a dry thickness 5μ, respectively,and baked in a direct hot gas furnace at the maximum plate temperatureof 200° C. for 60 seconds to obtain the test specimens.

The following tests were carried out with these specimens.

1. Continuous spot-weldability

Under the conditions of electrode diameter 5 mm, pressure 200 Kg,charged times 10 cycles, and electric current 8000A, spot-weldings werecarried out and the number of continously welded spots was determined.

Evaluation:

⊚ : more than 5000 spots, no contamination of electrode

○ : more than 5000 spots, considerable contamination of electrode

Δ: 3000 to 5000 spots

×: less than 3000 spots

2. Processability

Using a cylinderical cup drawing test apparatus (Model B1-142,manufactured by Erichsen Co.), the test specimen was processed under theconditions:

sheet holder pressure 3 tons,

punch diameter: 50 mm φ

dies diameter: 52.4 mm φ

draw depth: 40 mm

blank diameter: 95 mm

and film defect and peeling were examined.

Evaluation:

⊚ : no abnormality in coating

○ : less than 10% film defects (not reached to substrate metal)

Δ: many film defects, some of the defects being reached to the substratemetal

×: cracks of substrate plate

3. Anti-corrosive property

(a) Composite anti-corrosion test

1 cycle=salt spraying (5% Nacl, 35° C.) for 2 hours, drying (60° C.) for2 hours, and wetting (98% RH, 50° C.) for 4 hours

After repeating 200 cycles, generations of red rust and white rust onflat portion were examined. Evaluation:

⊚ : no generation of white and red rusts

○ : white rust generated area less than 5% no generation of red rusts

Δ: white rust generated area more than 6% no generation of red rusts

×: red rust generated area more than 1%

(b) salt spraying test (JIS-Z 2371)

After 2000 hours salt spraying, generations of red rust and of whiterust on flat portion were examined.

Evaluation:

⊚ : no generation of both white and red rusts

○ : white rust generated area less than 5% no generation of red rust

Δ: white rust generated area more than 6% no generation of red rust

×: red rust generated area more than 1%

The test results are shown in Table 9.

                  TABLE 9                                                         ______________________________________                                                      cylindrical                                                                             composit                                              contin. spot  cup draw  anti-corros.                                                                            salt spray                                  weldability   processabil.                                                                            test      anti-corros.                                ______________________________________                                        Example                                                                       31      ⊚                                                                        ⊚                                                                        ○                                                                              ⊚                          32      ⊚                                                                        ⊚                                                                        ○                                                                              ⊚                          33      ⊚                                                                        ⊚                                                                        ○                                                                              ⊚                          34      ⊚                                                                        ⊚                                                                        ⊚                                                                      ⊚                          35      ⊚                                                                        ⊚                                                                        ⊚                                                                      ⊚                          36      ⊚                                                                        ⊚                                                                        ⊚                                                                      ⊚                          Comp. Ex.                                                                     14      ○  ⊚                                                                        X       Δ                                   15      ⊚                                                                        ⊚                                                                        X       ○                                  16      ⊚                                                                        ⊚                                                                        X       Δ                                   ______________________________________                                    

EXAMPLE 37

Zinc nickel alloy electroplated steel sheet was coated by roll-coatmethod with the following chromate liquid, dried in a gas furnace for 1min. (maximum plate temperature 150° C.) to obtain a chromate coating(60 mg/m² chromium coverage).

As the chromate liquid, use was made of the solution obtained by mixingzinc potassium chromate having a solubility of 6 to 8 (15 g as metallicchromium), polyacrylic acid (Jurymer AC-10L, manufactured by NihonJunyaku K.K.) (10 g as solid) and water (to make the total volume to 11), and subjecting to a glass bead dispersion in a paint shaker for 1hour.

Separately, the following coating composition was prepared. 16 parts byweight (as solid) of urethane modified epoxy resin (prepared by themethod of Example 3 of Japanese patent application Kokai No. 30717/82,benzene ring content being 45 wt%) and 4 parts by weight (as solid) ofresol type phenol resin (BKS-316, manufactured by Showa Union GoseiK.K., benzene ring content being 55 wt%) were dissolved incyclohexanone. To this, were added 60 parts by weight of Ferrophos HRS2132 (electrically conductive pigment essentially of di-iron phosphide,manufactured by Hooker Chemicals and Plastic Co.) and 5 parts by weightof strontium chromate N (chromate pigment, iron phophide decompositioninhibitor, manufactured by Kikuchi Shikiso K.K.) and the mixture wassubjected to a steel bead dispersion in a paint shaker for 2 hours. Thedispersion was filtered, added with 15 parts by weight of Ceridust3620(finely pulverized polyethylene wax, manufactured by Hoechst AG),uniformly mixed in Disper and adjusted to non-volatile content 60 wt %by cyclohexanone.

To the abovesaid chromated steel sheet was applied the said coatingcomposition by roll-coat technique and baked in a gas furnace (maximumplate temperature 200° C.) for 1 minute, thereby obtaining ananti-corrosive coated laminate having a paint layer of 7μ thickness.

EXAMPLES 38 TO 43 AND COMPARATIVE EXAMPLES 17 TO 20

Using the same method as stated in Example 37 but varying the metalsubstrates, compositions of anti-corrosive layers and paint compositionsas given in Table 10, various coated laminates were prepared.

The chromate treating liquid containing water soluble chromium compoundhaving a solubility of more than 20 used in Example 38, was prepared asfollows:

Chromium trioxide (20 g as metallic chromium) was dissolved in 500 ml ofwater and the solution was added with formalin, thereby reducing 40% ofthe total chromium contained to Cr³ to obtain a partially reducedchromic acid aqueous solution. Next, 15 g of Aerosil 300 (fumed silica,manufactured by Nihon Aerosil K. K.) and zinc potassium chromate havinga solubility of 6 to 8 (15 g as metallic chromium) and water (in anamount enough to make the total volume to 1 1) were added and themixture was subjected to a glass bead dispersion in a paint shaker for 1hour.

Continuous spot-weldability, processability and anti-corrosive propertyof the respective laminate were evaluated and the test results wereshown in Table 11.

                                      TABLE 10                                    __________________________________________________________________________    Example   38         39        40         Comp. Ex. 17                                                                            41                        __________________________________________________________________________    substrate Zn/Ni alloy                                                                              Zn/Ni alloy                                                                             chromated, Zn/                                                                           Zn/Ni alloy                                                                             Zn/Ni alloy               material  elect. plated                                                                            elect. plated                                                                           Ni alloy ele. pl.                                                                        elect. plated                                                                           elect. plated             Cr compound                                                                             ZPC        ZPC 5     ZPC 5                ZPC                       A.sub.1 (g Cr/l)                                                                        (15)       Pb chromate 5                                                                           Pb chromate 5                                                                            no        15                                  part. reduct.                                                                            Ca chromate 5                                                                           Ca chromate 5                                                                            part reduct.                        A.sub.2 (g Cr/l)                                                                        CrO.sub.3  part. reduct.                                                                           part. reduct.                                                                            CrO.sub.3 part. reduct.                       (20)       CrO.sub.3 20                                                                            CrO.sub.3 20                                                                             40        CrO.sub.3 20              binder    Aerosil 300                                                                              Aerosil 300                                                                             Aerosil 300                                                                              Aerosil 300                                                                             Aerosil 300               (NV g/l)  15         15        15         15        15                        Cr content mg/m.sup.2                                                                   A.sub.1 60 A.sub.1 60                                                                              A.sub.1 60 A.sub.2 160                                                                             A.sub.1 60                anti-cor. layer                                                                         A.sub.1 + A.sub.2 140                                                                    A.sub.1 + A.sub.2 140                                                                   A.sub.1 + A.sub.2 140                                                                              A.sub.1 + A.sub.2                                                             140                       resin     B + C      B + C     B + C      B + C     B + C                               B/C = 8/2  B/C = 8/2 B/C = 8/2  B/C = 8/2 B/C = 8/2                 benzene ring                                                                            47         47        47         47        47                        content wt %                                                                  amount wt %                                                                             20         20        20         20        20                        elect. cond.                                                                            Ferrophos  Ferrophos Ferrophos  Ferrophos Ferrophos                 pigment   HRS 2132   HRS 2132  HRS 2132   HRS 2132  HRS 2132                  amount wt %                                                                             60         60        60         60        60                        polyethylene                                                                            Ceridust   Ceridust  Ceridust   Ceridust  Ceridust                  wax       3620       3620      3620       3620      3620                      amount wt %                                                                             15         15        15         15        15                        iron phosphide                                                                          strontium  strontium strontium  strontium strontium                 decomp. inhibit.                                                                        chromate N chromate N                                                                              chromate N chromate N                                                                              chromate N                amount wt %                                                                              5          5         5          5         5                        total pig. wt %                                                                         65         65        65         65        65                        film thick. μ                                                                         7          7         7          7        15                        __________________________________________________________________________    Example   42         43        Comp. Ex. 18                                                                             Comp. Ex. 19                                                                            Comp. Ex.                 __________________________________________________________________________                                                        20                        substrate Zn/Ni alloy                                                                              Zn/Ni alloy                                                                             Zn/Ni alloy                                                                              Zn/Ni alloy                                                                             Zn/Ni alloy               material  elect. plated                                                                            elect. plated                                                                           elect. plated                                                                            elect. plated                                                                           elect. plated             Cr compound                                                                             ZPC        ZPC       ZPC        ZPC       ZPC                       A.sub.1 (g Cr/l)                                                                        15         15        15         15        15                                             part. reduct.                  part. reduct.             A.sub.2 (g Cr/l)                                                                        part. reduct.                                                                            CrO.sub.3 part. reduct.                                                                            part. reduct.                                                                           CrO.sub.3                           CrO.sub.3 20                                                                             20        CrO.sub.3 20                                                                             CrO.sub.3 20                                                                            20                        binder    Aerosil 300                                                                              Aerosil 300                                                                             Aerosil 300                                                                              Aerosil 300                                                                             Aerosil 300               (NV g/l)  15         15        15         15        15                        Cr content mg/m.sup.2                                                                   A.sub.1 60 A.sub.1 60                                                                              A.sub.1 60 A.sub.1 60                                                                              A.sub.1 60                anti-cor. layer                                                                         A.sub.1 + A.sub.2 140                                                                    A.sub.1 + A.sub.2 140                                                                   A.sub.1 + A.sub.2 140                                                                    A.sub.1 + A.sub.2                                                                       A.sub.1 + A.sub.2                                                             140                       resin     B + C      B + C     B + C      B + C     D + C                               B/C = 8/2  B/C = 8/2 B/C = 8/2  B/C = 8/2 D/C = 9/1                 benzene ring                                                                            47         47        47         47        54                        content wt %                                                                  amount wt %                                                                             15         20        24         15        20                        elect. cond.                                                                            Zn powder  Ferrophos Ferrophos  Ferrophos Ferrophos                 pigment              HRS 2132  HRS 2132   HRS 2132  HRS 2132                  amount wt %                                                                             75         30        70         10        60                        polyethylene                                                                            Ceridust   Ceridust  Ceridust   Ceridust  Ceridust                  wax       3620       3620      3620       3620      3620                      amount wt %                                                                             10         15         1         15        15                        iron phosphide                                                                          strontium  St chrom. 5 wt %                                                                        strontium  St chrom. 5 wt                                                                          Strontium                 decomp. inhibit.                                                                        chromate N Imsil A-108 30%                                                                         chromate N Imsil A-108 50%                                                                         chromate N                amount wt %                                                                              0         35         5         55         5                        total pig. wt %                                                                         75         65        75         75        65                        film thick. μ                                                                         7          7         7          7         7                        __________________________________________________________________________

                  TABLE 11                                                        ______________________________________                                                      cylindrical                                                                             salt spray                                                                              comp. corro.                                contin. spot  cup draw  anti-corros.                                                                            200 cycles                                  weldability   processabil.                                                                            2500 H    A    B*                                     ______________________________________                                        Example                                                                       37      ⊚                                                                        ⊚                                                                        ○                                                                              ○                                                                           ○                             38      ⊚                                                                        ⊚                                                                        ⊚                                                                      ○                                                                           ○                             39      ⊚                                                                        ⊚                                                                        ⊚                                                                      ⊚                                                                   ⊚                     40      ⊚                                                                        ⊚                                                                        ⊚                                                                      ⊚                                                                   ⊚                     41      ○  ⊚                                                                        ⊚                                                                      ⊚                                                                   ⊚                     42      ○  ○  Δ ○                                                                           ○                             43      ⊚                                                                        ⊚                                                                        ⊚                                                                      ⊚                                                                   ⊚                     Comp. Ex.                                                                     17      ⊚                                                                        ⊚                                                                        ○                                                                              ○                                                                           X                                    18      X         X         ⊚                                                                      ○                                                                           ○                             19      X         ⊚                                                                        ⊚                                                                      ⊚                                                                   ⊚                     20      X         ⊚                                                                        ⊚                                                                      ○                                                                           ○                             ______________________________________                                         A. after 1 week from coating                                                  B. after 2 months from coating                                           

What is claimed is:
 1. A metal coating pre-treating agent comprising atleast one hexa-valent chromium containing chromium compound having asolubility of 10⁻⁵ to 20 and binder which is compatible with thechromium compound, the content of said chromium compound being 10⁻¹ to100 g/l as metallic chromium and the content of said binder being 10⁻²to 500 g/l as solid content.
 2. The metal coating pre-treating agentaccording to claim 1 further containing a water soluble chromiumcompound in an amount of 10⁻¹ to 100 g/l as metallic chromium.
 3. Themetal coating pre-treating agent according to claim 2 wherein thehexa-valent chromium content is more than 10% by weight of the totalwater soluble chromium.
 4. The metal coating pre-treating agentaccording to claim 1 wherein the binder is a member selected from thegroup consisting of aqueous silica, aqueous resin and a mixture thereof.5. The metal coating pre-treating agent according to claim 1, whichcontains a reaction promotor selected from the group consisting ofphosphoric acid, fluoric acid, fluosilicic acid, fluoboric acid,fluozirconic acid, fluotitanic acid and a salt of any of said acids.